Frequency jittering is commonly applied in switching mode power supplies to improve EMI (Electro Magnetic Interference). Conventional frequency jittering method is always used with switching mode power supplies with PWM (Pulse Width Modulation) control. The conventional frequency jittering method processes a clock signal of the switching mode power supply with PWM control to realize frequency jittering. However, in switching mode power supply without clock signal, novel frequency jittering method is needed. For example, in switching mode power supply with PFM (Pulse Frequency Modulation) control, there is not clock signal. But when an input voltage and an output voltage are fixed, the switching mode power supply with PFM control suffers from EMI too. Thus, Frequency jittering method are still needed in switching mode power supply with PFM control even if there is no clock signal
As a result, there is a need for frequency jittering method used with switching mode circuit without clock signal.